feat(pickers): a list that failed to load offers Retry

A picker whose options failed to load said so, but the only way back was closing its dialog.
Combobox takes `onRetry`: a Retry beside the message for the pointer, and Enter in the field for
the keyboard (the panel is a listbox, which may hold options only, so the alert names Enter for
assistive tech). Every caller with an error state passes its query's refetch: both user pickers,
the channel picker and the two Plex pickers.

frontend/src/components/ShareDialog.tsx

@@ -47,6 +47,7 @@ function UserShare({ job }: { job: DownloadJob }) {
         users={list}
         loading={recipients.isLoading}
         error={recipients.isError}
+        onRetry={() => void recipients.refetch()}
         disabled={share.isPending}
         onPick={(u) => {
           if (!share.isPending) share.mutate(u.email);